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Science Question for Cracked


Harpo Marx

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In school now Im working on molecule synthesis and balancing equations, and it's tough as hell. I can't get it because the teacher sucks and the class is boring so boring, which leads me to believe that I must have some sitting-still problem... but that's another story.

Anyways, here's an example:

Li2 S

Help would be greatly appreciated.

thanks

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I'm foggy on what the actual question is.

Li (lithium), if it winds up in a molecular compound, will have a charge of +1. Sulfur, as the sulfide ion, has a charge of -2. So it takes two lithium atoms and one sulfur atom to make lithium sulfide, Li2S, which as a normal molecule has a charge of zero (+1+1-2=0).

If Li2S is dissolved in water (I forget whether it does), it will break back up into two lithium ions (Li+) and one sulfide ion (S-2).

 

Li2S-->2(Li+)+(S-2) is the equation.

 

If that's not what you mean, then clarify the question.

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I guess ultra-strictly speaking your teacher wants to see you write it out: 2Li + S --> Li2S.

 

To know how many of one go with how many of another, you just need to know your periodic table, and what ions typically have what charge.

 

All these are +1: Li, Na, K, H, NH4 (I could go on, those are the commoner ones)

All these are -1: CL, F, Br, I, NO3, etc. If you need more of these I'll keep listing.

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OK, I think I see what you need.

Balancing equations really comes down to this: you MUST account for every single atom involved on BOTH sides of the arrow.

 

Let's look at why that K + O2-->K2O isn't balanced.

On one side you have one K and two O's. On the other, there's two K's and one O. You somehow invented an extra potassium, and made one oxygen atom disappear. Not possible.

Oxygen is diatomic - always comes in pairs in its natural state. (If there's an unnatural state going on, the problem must mention this, or you should assume that O by itself must always come in pairs: O2. I assume you've already learned about who's diatomic: H2, O2, N2, Cl2, Br2, I2, F2.)

To fix the equation, you need to figure out where the other oxygen atom went. The reaction is potassium plus oxygen makes potassium oxide. All of the oxygen is going to join up with potassium to make K2O, and you are forced to start with at least two oxygens, because it's diatomic. So there's gonna have to be two K2O's on the right. 2 molecules of K2O requires four K's, so you need to start with 4 K's on the left.

That sounds confusing. To balance an equation, you can monkey with the AMOUNTS on EITHER side of the arrow, or both. Here, when you're making potassium oxide, every oxygen atom needs two potassiums to go with it, period. You'd like to rewrite the equation 2K + O--> K2O. But you can't, because oxygen comes in twos. You can't write O by itself, it has to be O2, or 2(O2), or more. So if every oxygen needs two potassiums, and you're FORCED to start with two oxygens, you need four potassiums to balance it.

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That Fe(II) N2 looks shady. There are two possible interpretations, because there's two possible products.

Iron and nitrogen can conceivably form ferrous azide, Fe (N3)2, or ferrous nitride, Fe3N2. (Both of those are freaky, unstable compounds. My guess is they mean the azide, which is slightly more stable, I think.) The azide group is N3-1 (three nitrogen atoms globbed together with a total charge of -1). The nitride is N-3 (one nitrogen atom with a charge of -3). The iron valence they're using is two, or Fe+2.

 

For the azide, you have Fe+2 getting together with N3-1. So it'll take two azides for each iron. The tricky part is, nitrogen is diatomic, but each azide group has three nitrogen atoms in it, AND each ferrous azide molecule has six nitrogen atoms, so it's a chore to run down the math. I get:

 

Fe(II) + 3(N2)-->Fe(N3)2

 

For the nitride, you have Fe+2 getting together with N-3. The smallest molecule you can make with a charge of zero is Fe3N2: three irons and two nitrogens. That works out well because you were gonna have to start with two nitrogens anyway (the diatomic thing). So:

 

3Fe(II) + N2-->Fe3N2

 

I hope they're spelling out what the product of the reaction is going to be, rather than making you predict it.

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You probably stopped needing my input a while ago, but I love chemistry and was happy to jabber about it for two hours.

As an aside, I thought I'd give a little background on azides (N3-1). The azide ion is basically three nitrogen atoms in a triangle, with their hands around each other's throats. They're not happy, and not stable, and would much rather go back to being in pairs (N2). It doesn't take much to convince them. Azides are solids, and with the right provocation will explode. The explosion is basically the solid, compact N3 groups ripping apart and pairing off as gaseous N2 groups. A solid turning rapidly to a large volume of gas in a confined space is basically an explosion. Some azides will explode if they are hit with a hammer or other force. Which is why they are the active ingredient in auto airbags. In a forward collision, an extension of the part of the car getting hit slams into a pouch of azide compound, which explodes inside a big balloon, fast enough to inflate before your face can hit the steering column or dashboard.

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Guest HESHIANDET

cracked, we really haven't rapped too much but i know you have a hatred for organized society and the whatnot. All that aside, let me tell you about me. Im an observer, i look at things and determine weather or they're functional. i usually end up pointing out the faults though, hence my pesimistic views. where am i going w/ this? see, i liked chemistry, but due to a string of poor teachers and my own cynicism i dismissed chemistry for all it was worth. now today you come along and breakdown some fairly elementary chemistry and immediately sparked my interest in it again. I htink you'd be doing the world a favor if you were a teacher. i know that being a teacher usually ends up a redundant and downward spiral of a job- but hell, you made me want to sit down and tackle math problems....

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Originally posted by Cracked Ass

The azide ion is basically three nitrogen atoms in a triangle, with their hands around each other's throats. They're not happy, and not stable, and would much rather go back to being in pairs (N2). It doesn't take much to convince them. Azides are solids, and with the right provocation will explode. .

 

wow cracked, my field is geochemistry and all my years in studying and working with chemistry concepts, i've looked at things in a straight forward textbook manner, and never used imaginative metaphores like you have. all those nights of studying would of been so much easier.

 

back to work on my thesis...

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..and as an extra hint, balancing equations is sometimes a bit easier if you start with balancing the diatomics, usually oxygen and nitrogen, first..work with balancing the tougher parts of the equation first..

 

i don't have any of my old notebooks anywhere near me right now, so i can't give any examples right now, but this stuff gets way more complicated when you get into organic equations..multiple reactants, multiple products..

 

i had the easiest time by making a list of all the products on one side, all the reactants on the other..then everything is a checklist and you can balnce it out individually..rather than trying to fit each compound into a framework by guessing

 

..maybe tomorrow i'll give some good examples.

i love chemistry and i never get a chance to talk about it (especially on 12oz)

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..oh yeah..

if you're having problems with figuring out the products, the activity series helps..it shows which elements are more reactive than others.

 

more later on oxidation reduction balancing..very tough..

no matter can be created or destroyed, so the electrons have to be balanced as well as the atoms..

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Originally posted by HESHIANDET

cracked, we really haven't rapped too much but i know you have a hatred for organized society and the whatnot. All that aside, let me tell you about me. Im an observer, i look at things and determine weather or they're functional. i usually end up pointing out the faults though, hence my pesimistic views. where am i going w/ this? see, i liked chemistry, but due to a string of poor teachers and my own cynicism i dismissed chemistry for all it was worth. now today you come along and breakdown some fairly elementary chemistry and immediately sparked my interest in it again. I htink you'd be doing the world a favor if you were a teacher. i know that being a teacher usually ends up a redundant and downward spiral of a job- but hell, you made me want to sit down and tackle math problems....

 

This is exactly what I was thinking as I read this thread.

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Originally posted by bug

 

wow cracked, my field is geochemistry and all my years in studying and working with chemistry concepts, i've looked at things in a straight forward textbook manner, and never used imaginative metaphores like you have. all those nights of studying would of been so much easier.

 

back to work on my thesis...

 

have you ever heard of dr. von kukulo (i know i spelled it wrong)

 

he discovered the aromatic nature of the benzene ring..

after wrestling with the concepts (valence and double bonding, super stability)

he had a dream one night about snakes, chasing each other around in a circle, the head of one biting the tail of another..

this inspired him to come up with the alternating double bond stucture of the aromatic ring..

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His name was Kekule. I know the tale.

I have thought about offering my services as a tutor in chemistry. I enjoy rehashing the basics, and in that and in other areas I have a bit of a gift for choosing an analogy that the person I'm working with can relate to. In fact, I've thought about writing a sort of "chemistry for dummies" book which avoids the intimidating dryness of most science texts and instead goes for memorable images (like nitrogen atoms with their hands around each other's throats).

Sometimes I take the metaphors too far though. I always thought that oxygen difluoride had a homosexual character to it. Oxygen and fluorine are the two most electronegative (and reactive) elements, but fluorine is slightly stronger, so it can do to oxygen what oxygen normally does to everything else (oxidize it). Oxygen difluoride (OF2) reminds me of two big dudes who are in jail for long stretches, and both are so sex starved they turn gay. Fluorine is slightly more powerful and packs oxygen's fudge, even though oxygen is strong enough to make any other element do the pillow-biting.

Of course, releasing oxygen difluoride from containment is like letting those dudes out of jail. They'll split up and rape anything they see, grabbing electrons from whatever other element or compound is available on the outside. OF2 is a powerful oxidizer and will react with almost anything. It is exceptionally poisonous and burns the eyes, skin and lungs and causes intractable (incurable) headaches.

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Originally posted by Are2

..oh yeah..

if you're having problems with figuring out the products, the activity series helps..it shows which elements are more reactive than others.

 

more later on oxidation reduction balancing..very tough..

no matter can be created or destroyed, so the electrons have to be balanced as well as the atoms..

 

Actually in my experience using oxidation #'s to balance an equation is much easier ... the reason why is because there is pretty much zero guess and check work that you have to do with, umm, regular (for lack of a better word) balancing. The equation balances itself the first time with oxidation #'s.

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